Pharmacokinetics of bendroflumenthiazide in hypertensive patients

Hydrochlorothiazide is not the most useful nor versatile thiazide diuretic

PURPOSE OF REVIEW

To determine usefulness and versatility of hydrochlorothiazide (HCTZ) relative to other thiazide diuretics in the treatment of hypertension.

RECENT FINDINGS

HCTZ was found to be less potent in lowering blood pressure (BP) than other thiazide diuretics, including chlorthalidone (CTD) and bendroflumethiazide. A recent meta-analysis also suggested HCTZ (12.5-25 mg daily) to be less potent than antihypertensive agents from several other classes, including angiotensin-converting enzyme inhibitors, angiotensin-receptor blockers, and calcium antagonists. The risk of hyponatremia, hypokalemia, and hyperuricemia associated with HCTZ was lower than with CTD, while the risk of gouty arthritis was similar. Despite lower risks of metabolic side-effects, meta-analysis of clinical trials showed that, for any given difference in achieved clinic SBP, HCTZ therapy was associated with 18% higher adverse cardiovascular events when compared with CTD.

SUMMARY

Increasing evidence suggests inferiority of HCTZ in lowering BP and cardiovascular outcomes in hypertensive patients when compared with other drugs in the same class, particularly CTD and indapamide. Thus, HCTZ is neither more useful nor more versatile than other thiazide diuretics. CTD and indapamide should be preferred over HCTZ in most hypertensive patients when diuretics are required for treatment of hypertension.

Aspects on pharmacokinetics of some diuretics

This review summarizes the present knowledge of some commonly used diuretics. Bendroflumethiazide and bumetanide are completely absorbed from the gut while the uptake of hydrochlorothiazide, chlorthalidone and furosemide averages about 65%. The degree of uptake of amiloride and spironolactone is unknown but exceeds 50%. Plasma t 1/2 of bumetanide and furosemide are approximately 1 h. The clinically important phase of the plasma concentration of bendroflumethiazide has a t 1/2 of 3 h, although a slower phase with a t 1/2 of 9 h has been described. Hydrochlorothiazide and amiloride, often used in combination, both have a t 1/2 of about 10 h. Canrenone, an active metabolite of spironolactone, has a t 1/2 of 15-20 h. Chlorthalidone is eliminated very slowly with a t 1/2 of about two days. This is partly caused by an extensive binding to carbonic anhydrase in the erythrocytes. The protein binding of bendroflumethiazide, bumetanide, canrenone and furosemide is approximately 95%. The binding of chlorthalidone and hydrochlorothiazide is about 75 and 40% respectively. All mentioned diuretics except spironolactone are in part eliminated renally, mainly via tubular secretion. This is the major elimination route for amiloride and hydrochlorothiazide, while it constitutes one third to two thirds for bendroflumethiazide, bumetanide and furosemide. Spironolactone is exclusively eliminated as metabolites.

Comparison of slow-release piretanide and bendroflumethiazide in the treatment of mild to moderate hypertension

After 4 weeks of placebo treatment, 76 hypertensive patients were randomly allocated to 6 or 12 mg/day piretanide, or 2.5 mg/day bendroflumethiazide for 12 weeks in a double-blind study. Piretanide was given in a slow-release formulation and bendroflumethiazide as a tablet. All three treatments produced a significant reduction in supine and erect systolic and diastolic blood pressures after 2 weeks, and this effect was maintained throughout the study. Normotension (i.e. supine diastolic pressure less than or equal to 95 mmHg) was achieved in 73% of the patients receiving 12 mg/day piretanide and in 57% receiving 6 mg/day piretanide compared with 72% receiving bendroflumethiazide (not significant). Overall, five patients were withdrawn due to increased diuresis: two patients on each dosage of piretanide and one receiving bendroflumethiazide. Three patients receiving 6 mg/day piretanide were withdrawn due to diastolic blood pressure rising above 120 mmHg. Other side-effects reported were mild and transient. There were no significant changes in serum creatinine, glucose or high-density lipoprotein cholesterol. A small, but non-significant rise in uric acid level was seen in all three groups. Clinically relevant hypokalaemia requiring potassium supplementation occurred in three patients receiving bendroflumethiazide.

Bioavailability of bemetizide and triamterene from a combination formulation

The bioavailability of the thiazide diuretic bemetizide from a tablet containing 25 mg of this drug and 50 mg of the chemically unrelated diuretic triamterene was lower than, and significantly different (p less than 0.01) from that from a tablet containing 25 mg bemetizide alone. The mean peak plasma level of bemetizide after administration of the combination tablet (68.3 ng ml-1) was lower than that after administration of bemetizide alone (87.9 ng ml-1), although the times of occurrence of the peak levels were similar. The bioavailability of triamterene from the combination tablet was greater than, but not significantly different from that after administration of a capsule containing 50 mg triamterene alone. The mean peak plasma level of triamterene after administration of the combination tablet (44.6 ng ml-1) was higher than and significantly different (p less than 0.001) from that after administration of triamterene alone (15.7 ng ml-1). Although bemetizide is unstable in urine, measurement of the apparent excretion of unchanged drug in the 24 h post-dose urine (less than 4 per cent of the dose) agreed with the estimate of drug bioavailability from the plasma level data. Less than 2 per cent of the dose of triamterene was excreted unchanged in the 24 h post-dose urine, but the urinary excretion data also agreed with the bioavailability estimates from the plasma level data. The results of this study and those reported in the literature suggest that because of their physicochemical properties, the bioavailability of some thiazides and triamterene needs to be evaluated when new formulations of these drugs are produced. However, with respect to the combination formulation reported in this paper, the difference in bioavailability of the thiazide component did not detectably effect the diuretic activity of the formulation.

Pharmacokinetics of bendroflumethiazide alone and in combination with propranolol and hydralazine

Bendroflumethiazide (Bft) was administered to 6 healthy subjects at 3 different dose levels (2.5, 5 and 10mg) in a cross-over design, either as capsules (2.5mg) or as tablets (5mg). Its pharmacokinetics were evaluated then and following administration of a fixed combination of Bft and propranolol and hydralazine to a further 7 volunteers. Plasma and urinary concentrations of Bft were determined by a new fluorimetric - thin-layer chromatography procedure. Peak plasma levels occurred after 2-3h and averaged 15, 27 and 45 microgram/l in the three dose groups. Areas under the plasma concentration - time curves (AUC0 leads to 12), which were 75, 147 and 250 microgram 1(-1)h respectively, and cumulative urinary recovery (20%) were independent of the dose administered and the type of formulation. Thus Bft kinetics proved to be linear within the dose range evaluated. The plasma clearance was calculated to be 505 ml/min, renal clearance 108 ml/min and nonrenal clearance 396 ml/min. Bioavailability of Bft was not altered following administration of the fixed combination. The amount of propranolol found in the circulation did not change, whereas that of hydralazine (determined as apparent hydralazine) increased by 59% when the fixed combination was administered.

Pharmacokinetics of bendroflumethiazide after low oral doses

The pharmacokinetics of bendroflumethiazide after oral administration of 1.25, 2.5, and 5.0 mg were studied in nine healthy male volunteers. Bendroflumethiazide was analyzed by GLC after extractive alkylation. After the lowest dose, the plasma concentration, could be followed to 14 hr, and the data were adequately fitted by a one-compartment model; the half-life was 3.1 hr. After the 2.5 and 5.0 mg doses, the plasma concentration was followed for 24 hr, and the data were fitted by a two-compartment model with half-lives of 8.9 hr. The urinary sodium concentration was doubled after bendroflumethiazide intake, but the urinary potassium concentration remained almost constant. The renal clearance of bendroflumethiazide was around 30 ml . min-1.

Antihypertensive effect of various doses of hydrochlorothiazide and its relation to the plasma level of the drug

Nine previously untreated hypertensive patients (WHO Stage I and II) were given hydrochlorothiazide (hct) 12.5, 25, 50 and 75 mg for two weeks after an initial four week period of placebo treatment. Blood pressure recordings were made casually and after 30 min rest in lying, sitting and standing positions. Plasma concentrations of the drug were measured by GLC. A significant decrease in BP was seen during treatment with hct 12.5 mg except in the casual standing position. Doubling of the dose twice produced very little further decrease in BP. The mean steady state concentration was 111 ng.ml-1 during treatment with hct 75 mg. A linear relationship was found between the plasma concentration of hct at 0 and 5 h and the various dose level of hct. No relation was found between plasma concentration and reduction in blood pressure.